Radar-based methods are since several years commonly used for level gauging in various tanks. The antenna devices used for transmitting and receiving the radar signals, are critical parts and a few different antenna types have been employed.
Three basic factors to consider for an antenna for radar-based level gauging are gain, antenna pattern, and size.
The antenna gain, which is closely related to the efficient antenna area, has to be high enough to provide for receiving a sufficiently strong signal echo from the required maximum distance. For a circular antenna the maximum measuring distance is, in the limiting case where the liquid surface is turbulent, proportional to the antenna diameter.
Further, the radiation beam has to be narrow enough to suppress echoes from obstacles in the tank in order to distinguish the echo from the surface of the matter gauged. Since the antenna pattern is also related to the antenna area, the requirement of a narrow radiation beam can, for a circular antenna, be reformulated to a requirement of an antenna having sufficiently large diameter.
Still further, the antenna has typically to be small enough to be inserted through an opening in the roof of the tank while being mounted. Using an opening at some arbitrary position might not be possible since locations where internal tank structures may give disturbing radar echoes have to be avoided. For many tanks, such as tanks containing inflammable liquids there may also be restrictions on having new openings welded, and then a suitable existing opening has to be used.
Obviously, the three requirements may not always be complied with, e.g. when the first two requirements imply an antenna size, which is larger than the only suitable opening.
Typical antennas for radar level gauging are horn, planar, parabolic and rod antennas. When the available tank roof opening is small an end-fire rod antenna or a very small horn antenna is typically used today, or the last one of the three requirements may be removed if the antenna can be mounted from the inside of the tank. This, however, obviously requires a tank environment allowing a person to enter for the necessary mounting work.
The use of a horn antenna for 4″ and smaller tank roof holes is described e.g. in U.S. Pat. No. 6,404,382 issued to Fehrenbach et al.
The end-fire rod antenna may typically at 6 GHz be a shaped PTFE-bar having a length of about 300-350 mm and a diameter of about 30 mm or smaller. The antenna has an efficient area, which is larger than the geometrical cross section. The rod antenna roughly corresponds to a 3″ horn antenna, which has a 3 dB lobe width of 30°. Due to the end-fire nature of the rod antenna this is hard to improve without using very long rods. The gain of an end-fire antenna is proportional to its length as compared to the gain of a broadside antenna, such as horn or parabolic antenna, which is proportional to the area of the antenna.
Long rods, however, limit the possibility of gauging high levels, i.e. levels close to the roof of the tank, since the lowermost portion of the rod would then be below the surface of the gauged matter.
Another solution is to use a parabolic foldable antenna as described in U.S. Pat. No. 5,926,152 issued to Schneider. For the purpose of introducing the measuring instrument through an opening in the container, the parabolic antenna can be folded and unfolded by being positively moved.
The parabolic foldable antenna as disclosed in said U.S. Pat. No. 5,926,152, however, seems to need a rather large tank roof opening. Further, the foldable parabolic antenna seems to be a complex device, which would be expensive to manufacture.